Lean premixed combustion in gas turbine engines tends to be susceptible to combustion driven oscillations such as dynamic instability. The risk of such instability increases as the flame temperature in such lean premixed systems is continually lowered to reduce NOx emissions. Flame temperatures have been driven near to the lean blowout limit, at least with fuels having a high methane content. For such lean mixtures, slight variations in the local fuel to air mixture ratio may result in periodic variations in the local heat released and the heat release rates. Discrete oscillation frequencies or tones can grow in amplitude when the heat release fluctuations are constructively in phase with the acoustic pressure fluctuations encountered inside the combustion chamber.
As the present lean premix combustors become leaner and more spatially uniform, the risk of encountering unacceptably high levels of dynamics may go up. Combustion dynamics typically have been abated by shifting the fuel injection points to alter the fuel transport time from the point of injection to the flame front, by changing the fuel injection orifice sizes to alter the pressure drop and the acoustic impedance across the apertures, or by modifying the chamber or nozzle geometries to affect vortex shedding, frequencies and amplitudes, or flame shape. These abatement efforts attempt to force any perturbations in the heat release to be out of phase (or destructively in phase) with the pressure or acoustic perturbations in the combustion chamber. Adding acoustic dampening to the combustion system also has reduced combustion dynamics. These methods, however, usually involve trial and error and can be expensive and uncertain.
There is a desire, therefore, for an improved lean premixed combustion system that meets applicable NOx emission regulations while reducing or eliminating dynamic instability. The system preferably maintains overall engine efficiency and provides tolerance to the fuel mixture quality.